This invention is generally in the field of optical devices and relates to a method and device for the tunable frequency selective filtering of optical signals, particularly useful for adding or dropping channels in a wavelength division multiplexing optical communication system.
Optical transmission systems, which are based on wavelength division multiplexing (WDM), achieve high information capacities by aggregating many optical channels onto a single strand of optical fiber. Tunable filters play a critical role in WDM communication systems. A tunable filter, which can redirect and route wavelengths, is used in conjunction with tunable lasers to create a tunable transmitter, midway in the fiber in wavelength for add and drop multiplexing applications, and at the receiving end in conjunction with a broad band detector for a tunable receiver.
In applications of add and drop multiplexing, the tunable filter is often termed a three (or more) port device, with an input, express, and drop (add) ports. In these applications, the network traffic enters the device at the input, with most of the channels leaving at the express port. The dropped channels are redirected to the drop port, while the added channels are input from the add port. During all times, the network is operational, and in particular, when tuning the filter from one channel to another, a critical feature of the filter is termed xe2x80x9chitless tuningxe2x80x9d, which is the ability to tune from one channel to another without disturbing (xe2x80x9chittingxe2x80x9d) any of the express channels, since this would constitute a traffic disruption in the network.
Tunable filters in state of art implementations fall under the following two categories:
(1) Tunable filters based on spatial distribution of the different channels and switching of the channels to be dropped. Here, tunability is achieved by applying spatially distinct switches, which switch different channels to the drop port.
(2) Tunable filters based on a change in the frequency of operation by physical changes in the optical filter medium. These are the so-called xe2x80x9cscanningxe2x80x9d tunable filtersxe2x80x9d, since they scan over frequencies.
Hitless tuning can easily be achieved in the first implementation. However, the first implementation suffers from many other drawbacks, especially energy loss, cross talk, and price, all of which make its use difficult for optical networks. The second type of filter is a preferred solution for most optical networks.
U.S. Pat. No. 6,292,299 describes a hitless wavelength-tunable optical filter, which includes an add/drop region and a broadband optical reflector adjacent thereto. The operation of the filter is based on selectively repositioning an optical signal in the add/drop region while adding or dropping an optical wavelength channel, and on the use of a broadband optical reflector, while tuning to a different optical wavelength channel.
xe2x80x9cAll fiber active add drop multiplexerxe2x80x9d [IEEE Photonics Technology Letter, Vol. 9, No. 5 p 605] describes an architecture to be used as a reconfigurable router for exchanging channels between two fibers or as a reconfigurable add/drop multiplexing filter. The architecture consists of a Mach-Zender interferometer with identical gratings written in each arm, one pair of grating for each wavelength to be added or dropped. Each grating pair is also accompanied by a phase shifter, which is a thermo-optic heater.
There is a need in the art to facilitate the tuning of a frequency selective filter by providing a novel optical method and device for continuously flowing light through the frequency selective filter. The frequency selective filter may perform wavelength dropping or adding function.
The present invention utilizes separating at least a portion of the power of a selected frequency component from the remaining portion of the multi-frequency input light signal and directing the separated light components along two spatially separated optical paths, creating a phase delay between these optical paths by affecting the phase of the light component of said at least portion of the selected frequency band. This enables to either direct all the frequency components of the input light to a common first output channel with no power in a second output channel, or direct the entire power of the selected frequency band and all other frequencies along, respectively, the second and first output channels, depending on the phase delay between the two spatially separated optical paths. Thus, on one operational mode of the device according to the invention, all the input light is output in one channel, while the other output channel serving for dropping or adding function is inoperative, and in the other operational mode of the device, the selected frequency band is fully spatially separated from all other frequencies, and can therefore be either dropped or added to another optical signal.
The above is implemented by passing the input light through a first tunable frequency coupling element having two input ports, of which one is used for receiving the input multi-frequency light and the other is unconnected. The coupler further has two output ports associated with two spatially separated optical paths (waveguides). The optical path difference between the spatially separated waveguides can be adjusted by various well-established means. The phase difference can be adjusted between zero path difference, both waveguides having exactly the same optical length, and 1800 path difference, the optical length difference between the waveguides being equal to half the wavelength. The two waveguides are input into a second, reciprocal frequency-coupling element, which has two inputs and two outputs. The light input from both ports is recombined in the coupler, whereas in the first coupler, only one input port was active and in the second coupler both input ports are active and the combination of the two ports in the second coupler depends on the relative phase of the input ports.
The phase delay between the two spatially separated optical paths can be continuously adjustable up to 180xc2x0. The output at the second coupling element depends in a continuous manner on the phase delay between the two spatially separated optical paths, such that for a zero phase delay between the optical paths, the tunable selected frequency band of the input light is in one output channel of the device, and the remaining spectral content is in the other output channel, while for a 180xc2x0 phase delay, substantially all the input light is in the same output channel.
At intermediate phase states, the amount of light at the selected frequency band is selectively variable. Selective dropping of a portion of the energy of a given frequency band is known as xe2x80x9coptical broadcast functionalityxe2x80x9d and is useful in instances where the optical signal has to reach more than one destination node.
There is thus provided according to one aspect of the present invention, a method for controlling continuous propagation of input multi-frequency light through a tunable frequency selective optical filter device so as to selectively direct a selected frequency band of the input light to a dropping/adding output channel of the device, the method comprising:
(i) applying selective frequency coupling to the input light to split the input multi-frequency light into first and second light components propagating through first and second spatially separating optical paths, respectively, the first light component comprising at least a portion of power of the selected frequency band of the input light, and the second light component comprising a remaining portion of the selected frequency band and all other frequency bands of the input light;
(ii) selectively creating a phase delay between the first and second optical paths by adjusting the phase of said first light component;
(iii) depending on the phase of said first light component, either combining the first and second light components to propagate through a first output channel with substantially no power in the second dropping/adding output channel, or directing all the power of the selected frequency band through the second dropping/adding output channel while directing all other frequency components of the input light through the first output channel.
According to another aspect of the present invention, there is provided a method for controlling continuous propagation of input multi-frequency light through a tunable frequency selective optical filter device so as to selectively direct to a selected frequency band of the input light to a dropping/adding output channel of the device, the method comprising:
splitting the input multi-frequency randomly polarized light into first and second spatially separated components of orthogonal polarization directions and directing the first and second polarization components along first and second spatially separated channels, respectively;
applying a 90xc2x0 polarization rotation to either one of the two polarization components, thereby producing two identically linearly polarized components;
applying a selective frequency coupling to each of the two identically linearly polarized components to produce first and second light components propagating through first and second spatially separating optical paths, respectively, wherein the first light component comprises at least a portion of power of the selected frequency band of the input light, and the second light component comprises the remaining portion of the input light;
selectively creating a phase delay between the first and second optical paths by adjusting the phase of said first light component;
depending on the phase of said first light component, either combining the first and second light components to propagate through a first output channel with substantially no power in the second dropping/adding output channel, or directing all the power of the selected frequency band through the second dropping/adding output channel while directing all other frequency components of the input light through the first output channel.
According to yet another aspect of the present invention, there is provided a tunable frequency selective optical filter device operable to provide continuous propagation of input multi-frequency light through the device enabling to selectively direct a selected frequency band of the input light to a dropping/adding output channel of the device, the device comprising:
(a) a first tunable frequency coupling element having an input for receiving the multi-frequency input light and two outputs associated with two spatially separated optical paths, respectively, the first coupling element being operable to split the input light into two light components propagating through said first and second optical paths, respectively, the first light component comprising at least a portion of power of a selected frequency band of the input light, and the second light component comprising the remaining portion of the input light;
(b) a phase adjusting element accommodated in the first optical path and operable to affect the phase of light propagating therethrough, thereby adjusting a phase delay between the first and second optical paths;
(c) a second tunable frequency coupling element having two inputs associated with the two optical paths, respectively, and first and second outputs associated with first and second output channels, respectively, the second frequency coupling element being responsive to the phase delay to either combine the first and second light components to propagate through the first output channel, or direct all the power of the selected frequency band to the second dropping/adding output channel and direct all other frequency components of the input light to the first output channel.
Each of the first and second tunable frequency coupling elements can be realized using a grating assisted coupler (GAC) [xe2x80x9cGrating-Assisted Codirectional Coupler Filter Using Electrooptic and Passive Polymer Waveguidesxe2x80x9d, Seh-Won, Ahn and Sang-Yung Shin, IEEE Journal on Selected Topics in Quantum Electronics, Vol. 7, No. 5, September/October 2001, pp. 819-825] that transfers light of a specific frequency band from one output of the coupler to the other, or can be realized by using any other suitable coupler device, for example, of the kind whose physical parameters, such as the length, the strength of coupling between the two optical paths, and the phase difference across the coupling length, define the amount of transferred energy.